Add `caution` icon to icon set

This icon is an MIT-licensed SVG sourced from https://feathericons.com/

src/annotator/icons.js

@@ -7,6 +7,7 @@
 export default {
   annotate: require('../images/icons/annotate.svg'),
   cancel: require('../images/icons/cancel.svg'),
+  caution: require('../images/icons/caution.svg'),
   'caret-left': require('../images/icons/caret-left.svg'),
   'caret-right': require('../images/icons/caret-right.svg'),
   hide: require('../images/icons/hide.svg'),

src/images/icons/caution.svg

+<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2.5" stroke-linecap="round" stroke-linejoin="round" class="feather feather-alert-triangle"><path d="M10.29 3.86L1.82 18a2 2 0 0 0 1.71 3h16.94a2 2 0 0 0 1.71-3L13.71 3.86a2 2 0 0 0-3.42 0z"></path><line x1="12" y1="9" x2="12" y2="13"></line><line x1="12" y1="17" x2="12.01" y2="17"></line></svg>
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